79 



belong to the same botanical family as the Apple-tree. It is observ- 

 able, however, that aJ though Apple-trees and Peach-trees are com- 

 monly grown in the same orchard near Cobden, yet I did not hear 

 of a single case, where the roots or bark of the Peach-tree had been 

 found to be infested by this insect, or any other at all resembling it. 



Persons, who are not familiar with the habits and classification 

 of Insects, are apt to think that all Plant-lice — different as some of 

 them are in shape, color, size and habits — are mere varieties of one 

 and the same species; just as all dogs, however much they may differ 

 from one another in such respects, yet belong to one and the same 

 species. But it is by no means so. Whatever food one variety of 

 Dog will live on, another variety of Dog will live on equally well. 

 But shift the Apple-tree Plant-louse on to a Cherry-tree, or tha 

 Cherry-tree Plant-louse on to a Plum-tree, or the Plum-tree Plant- 

 louse on to a Peach-tree, and it will before many days die of starva- 

 tion. Yet these trees all belong to the same botanical Family. Dr. 

 Fitch has remarked that the imported species of Plant-louse, that 

 inhabits the tame Cherry-tree, cannot live even upon any of our 

 wild Cherry-trees, and that even the wild Black Cherry (Cerasus 

 serotina) and the Choke-cherry (Cerasus virginiana) , closely related 

 as they are, are inhabited by distinct species. {N. Y. Rep. I. p. 131.) 

 Again: the Currant and the Gooseberry belong to the same genus. 

 The Currant is notoriously infested by a Plant-louse (Aphis rihis, 

 Linngeus) which has been imported along with that shrub from 

 Europe. Yet, although these two plants are often grown in gardens 

 side by side, nobody ever saw any plant-lice of any kind on the 

 Gooseberry, much less the true Currant Plant-louse. Yet, if the 

 same species of Plant-louse can feed almost indiscriminately upon 

 any kind of plant, why does not the Currant Plant-louse emigrate 

 on to the Gooseberry? I have myself observed that different species 

 of Plant-lice (Aphis) inhabit dift'erent species of Oak; for example, 

 the Handsome Plant-louse (Aphis hella, Walsh) is peculiar to the 

 Black Oak, [Quercus tinctoria,) where, since I described the species, 

 I have found it in abundance in company with its larva ; and an un- 

 described species of the same genus, with remarkably stout branch- 

 veins to its front wings, is peculiar to the S^w^amp White Oak, (Quer- 

 cus prinus, var. discolor.) 



As regards the Eoot Plant-lice, we know but very little of the 

 species found in this country, because, like other underground insects, 

 they are hidden from our observation in the bowels of the earth. 

 Indeed, besides the species now under discussion, the only other de- 

 scribed North American species are two, which I myself was the first 



